1. Field of the Invention
This invention relates to a mineral carrier for oxides, advantageously used in packed beds for scavenging hydrogen sulfide and mercaptans from fluid streams such as natural gas, carbon dioxide, and liquid hydrocarbons.
2. Description of the Related Art
The wood chip process has for many years been used to scavenge hydrogen sulfide from gas streams. Describing this process generally, a stream of the polluted gas is driven through a bed of moistened wood chips containing a reactant iron oxide. While in widespread use, the process has numerous shortcomings, including that the amount of hydrogen sulfide removed is limited to the stoichiometric amount that would react with such Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4 oxides as are supplied to form pyrites, and also that the pressure of the gas stream suffers a severe drop as it passes through the bed, and that as the oxides react, they in effect cement the bed material into a unified mass. Removing the bed material from the reactor vessel is a difficult and time-wasting procedure; and the removed material is environmentally unsafe. Wood chips are not suited for liquid streams; they become sodden, impairing perviousness of the bed.
A greatly improved iron oxide was shown in U.S. Pat. No. 4,246,244. That oxide, whose particles are composed of a crystalline phase of Fe.sub.3 O.sub.4 together with an amorphous Fe.sub.2 O.sub.3 moiety, when suspended in a water slurry and reacted by hydrogen sulfide gas bubbled therethrough, creates a stable waste which after drying does not pollute the atmosphere. The surface area of that oxide is at least 4.0 m.sup.2 /g; when used in the "dry bed" process described, it has a kinetic "R" value in excess of 0.5.times.10.sup.-4 ft..sup.3 /(min.-gram oxide). At temperatures above approximately 150.degree. C. this improved oxide is destroyed. It is obtainable from Gas Sweetener Associates, St. Louis, Mo. That oxide is hereinafter referred to as the "special" or "preferred" oxide; it is the oxide utilized in all of the tests and discussions which follow except where use of other oxides is particularly pointed out.
U.S. Pat. No. 4,366,131 shows use of that oxide to sweeten gas in a dry process (analogous to the wood chip process) in a bed of inert particulate matter, there shown to be sand. While that patent shows that this special oxide will react effectively in a dry process, objectionable bed caking was encountered even though the bed was shallow. A partially effective expedient, leaving a substantial head-space above the bed and directing the gas upward through it to "fluidize" the bed, was likely to result in breakthrough of unscavenged gas.
Gases to be freed of hydrogen sulfide are often hydrocarbons which can also contain various amounts of mercaptans, carbon dioxide, oxygen, water, and other components. Substantially pure carbon dioxide from fermentation or other sources can also contain hydrogen sulfide. Most often, batch gas sweetening processes are adversely affected by the presence of carbon dioxide due to its tendency to lower pH.
While the foregoing patents were concerned with sweetening gas, a limited use of that oxide to react hydrogen sulfide polluted liquids is shown, for example, in U.S. Pat. No. 4,344,842 in which anhydrous kerosene was simply flowed in admixture or contact with the oxide particles. The addition of the particles to aqueous drilling muds, made corrosive by pollution with hydrogen sulfide, as well as oxygen, is taught in U.S. Pat. No. 4,634,539.
Other types of sweetening processes, when applied to liquids, have been generally unsatisfactory. For example, the wood chip process, widely used for sweetening gas, is not suited for contaminated liquids; the chips become saturated non-uniformly, causing premature channeling through the chip bed, low reaction efficiencies and unpredictable results. Amine processes are expensive, and it is difficult to separate the amines from liquid hydrocarbons.